The present invention relates to graphite powder used for negative electrodes of lithium ion secondary cells (batteries). More particularly, this invention relates to the graphite powder, which is capable of improving the discharge/charge efficiency and the discharge capacity of the cell and a method for producing the same.
As the material for a negative electrode of lithium ion secondary cells, carbon materials such as graphite and carbon are used in practice owing to the advantage in that the formation of needle crystals of lithium dendrite can be avoided during the charging and discharging. In order to adjust the particle size of the carbon mate rial to a level as desired, they are subjected to dry grinding in most cases. That is, they are ground by using a jet mill or the like to obtain their particles of a certain particle size by means of recovering with a cyclone. The obtained particles are then sieved to produce the particles in a certain range in particle size to be used. The particles of carbon material obtained by dry grinding have many active sites on the minute surfaces that are newly formed in the grinding. The carbon material having the above-mentioned active sites often cause the decomposition of electrolytes or the formation of irreversible lithium compound. For this reason, the high capacity and high reversibility required of the carbon material for lithium ion secondary cell are influenced adversely. In other words, the high Coulomb efficiency and the long-term storage life are adversely influenced; as a result, the characteristic properties of obtainable lithium ion secondary cells are impaired.
In order to solve these problems, the use of graphite material substantially containing none of fine graphite particles of 10 xcexcm or less in particle size is disclosed in Japanese Laid-Open Publication No. H06-52860. Furthermore, disclosed in Japanese Laid-Open Patent Publication No. H06-295725 is graphite material for negative electrodes, which graphite material has an average particle size in the range of 10-30 xcexcm and a specific surface area (BET value) in the range of 1-10 m2/g, and the quantities of graphite particles of less than 10 xcexcm and more than 30 xcexcm in particle sizes are limited to a value not more than 10%.
Furthermore, it is disclosed in Japanese Laid-Open Patent Publication No. H09-213335 to incorporate at least one element selected from the group of Mg, Al, Si, Ca, Sn and Pb into the carbon material for the negative electrode of lithium ion secondary cells by means of mixing the carbon material with at least one member of Mg2S, Al4C3, tin oxalate and CaC3 which is followed by the treatment of sintering.
Still further, disclosed in Japanese Laid-Open Patent Publication No. H09-249407 is a material for negative electrode of lithium cells, which material is prepared by forming mechano-chemically a graphite composite using graphite particles and solid element particles of Li, Al, Sn, Pb and Cd.
Furthermore, disclosed in Japanese Laid-Open Patent Publication No. H08-45548 is that the material for negative electrode of lithium secondary cell is added or plated with 3-10 wt. % of at least one kind of metallic powder element selected from the group consisting of gold, silver, copper, nickel and chromium.
However, in the carbon materials (including graphite) which are obtained by dry grinding, the formation of active site on the surfaces of particles cannot be avoided, as described above. Furthermore, in the case of the carbon material prepared by dry grinding, the carbon material substantially contains fine particles even when the particle size is controlled by sieving or cyclone recovering, because the particle size is not controlled in the stage of the primary particles. That is, in the fine particles of less than 1 xcexcm in particle size obtained by dry grinding have high surface energy so that the particles aggregate during the dry grinding process to form the secondary particles of several tens of xcexcm in particle diameter.
The present invention has been accomplished in order to solve the above-described problems. Several graphite materials such as natural graphite, artificial graphite, kish graphite, mesophase carbon micro-beads (MCMB), mesophase carbon micro-fiber (MCF) and resin carbonized graphite which can occlude and release lithium ions, are used as the graphite materials for negative electrodes of lithium ion secondary cells. The carbon material proposed by the present invention is prepared by causing the surfaces of graphite material to adsorb or to be coated by 0.01 to 10 wt. % (on the basis of graphite material) of a material having surface active effect (hereinafter referred to as xe2x80x9csurface active effect materialxe2x80x9d) which is at least one member elected from the group consisting of starch derivatives having a basic structure of C6H10O5, viscous polysaccharides having a basic structure of C6H10O5, water-soluble cellulose derivatives having a basic structure of C6H10O5 and water-soluble synthetic resins.
Furthermore, the present invention provides the graphite material for the negative electrodes of lithium ion secondary cells, in which the graphite powder is coated with or adsorbs the above-mentioned surface active effect material can further contain 50 to 30,000 ppm of at least one alkali metal element or alkaline earth metal element selected from the group consisting of lithium, calcium, magnesium, sodium and potassium.
Another aspect of the present invention is to provide a method for producing the graphite powder for negative electrodes of lithium ion secondary cells, which method comprises the steps of adding graphite powder into an aqueous solution of a surface active effect material; dispersing the mixture with stirring; then filtering and drying the mixture, thereby obtaining the graphite material, in which 0.01 to 10 wt. % on the basis of the graphite material, of a surface active effect materials are adsorbed or coated. The above surface active effect material is, as described above, at least one member selected from the group consisting of starch derivatives having a basic structure of C6H10O5, viscous polysaccharides having a basic structure of C6H10O5, water-soluble cellulose derivatives having a basic structure of C6H10O5 and water-soluble synthetic resins and the above graphite powder is exemplified by natural graphite, artificial graphite, kish graphite, mesophase carbon micro-beads (MCMB), mesophase carbon micro-fiber (MCF) and resin carbonized graphite which are able to occlude and release lithium ions.
In addition, by using the water containing lithium calcium, magnesium, sodium and potassium in the above method, the present invention further provides another method for producing graphite powder for negative electrodes of lithium ion secondary cells, in which the graphite powder contains 50 to 30,000 ppm of at least one alkali metal element or alkaline earth metal element selected from the group consisting of lithium, calcium, magnesium, sodium and potassium.
In this description, the above graphite powder used for negative electrodes of lithium ion secondary cells is sometimes simply referred to as xe2x80x9cgraphite powderxe2x80x9d and the ordinary graphite which is used as the raw material for the graphite powder of this invention will be referred to xe2x80x9cgraphite materialxe2x80x9d.
The graphite powder adsorbing or coated with various compounds in the present invention can be prepared by mixing graphite powder into an aqueous solution of the above compounds and by dispersing with stirring, which are followed by filtration and drying.
The graphite materials used in the present invention are exemplified by natural graphite, artificial graphite, kish graphite, mesophase carbon micro-beads (MCMB), mesophase carbon micro-fiber (MCF) and resin carbonized graphite. That is, they are optionally selected according to the structure of lithium ion secondary cell of the combination of positive electrode material, electrolyte and separators. Any of graphite materials which can occlude and release lithium ions can be used for the negative electrodes of lithium ion secondary cells.
The surface active effect materials to be adsorbed by or to be coated to the graphite material are exemplified by starch derivatives having a basic structure of C6H10O5 such as acetic starch, phosphoric starch, carboxymethyl starch and hydroxyalkyl starch e.g. hydroxyethyl starch; viscous polysaccharides having a basic structure of C6H10O5 such as pullulan and dextrine; water-soluble cellulose derivatives having a basic structure of C6H10O5 such as carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose; and water-soluble synthetic resins such as water-soluble acrylic resin, water-soluble epoxy resin, water-soluble polyester resin and water-soluble polyamide resin. The aqueous solution of the surface active effect material can be prepared by dissolving one or more of them into water.
When the above graphite material is added into the above aqueous solution and it is dispersed by stirring, the fine primary particles agglomerate into secondary particles and the secondary particles turns into primary particles, at the same time, the surface active effect material electrically and chemically covers or be adsorbed by a large number of active sites existing on the surfaces of the latter primary particles. Therefore, the whole surfaces of primary particles of graphite material are covered by the surface active effect material.
The quantity required for covering the whole surfaces of primary particles of graphite material depends upon the surface area of the graphite particles, however, the quantity is generally preferable in the range of 0.01 to 10 wt. % on the basis of the quantity of graphite material.
If the quantity of surface active effect material adsorbed by or coated to the graphite material is less than 0.01 wt. %, the object of the present invention cannot be attained because the quantity is too small for producing the surface active effect and the active sites on the graphite material cannot always be covered. When the adsorption quantity or coating quantity is too small, the graphite material in the aqueous solution cannot be dispersed well. So that, the evaluation of the quantity of surface active effect material can easily be done by observing whether the graphite material is floating or not in the treating liquid.
With the increase in the adsorption or coating quantity of the surface active effect material to the graphite material, the characteristic properties as the graphite powder for negative electrodes of lithium ion secondary cells are improved, however, when the quantity exceeds 10 wt %, the properties are impaired because the electro-conductivity inherent in the graphite material is lowered and the quantity of occlusion of lithium ions by the graphite powder is reduced.
The quantity of adsorption or coating of the surface active effect material to the graphite material can be adjusted by controlling the concentration of the aqueous solution. When the concentration is set too high, it is possible to adjust the quantity of adsorption or coating by rinsing the filter cake of graphite material with water.
Meanwhile, the adsorption or coating quantity of surface active effect material can be adjusted to a certain level by employing X-ray spectrophotometric analysis. That is, when graphite material without the treatment of adsorption or coating is subjected to X-ray spectrophtometry, the value of C1S is 95-100 atomic % and O1S, 0-5 atomic % in the surface atomic percentage of C1S and O1S. In the case of the graphite material which is treated by the adsorption or coating of surface active effect material according to the present invention, the value of C1S is 85-95 atomic % and O1S, 5-15 atomic %. This is due to the functional groups of carboxylic groups, carboxylic acid groups, ester groups and hydroxyl groups contained in the surface active effect material existing on the surface of graphite powder.
It is possible to improve the discharge capacity of lithium ions when the graphite material treated with the surface active effect material further contains the aforementioned element of alkali metal or alkaline earth metal. The effective quantity of the above element for the improvement in discharge capacity is 50 to 30,000 ppm. When the quantity of such element is less than 50 ppm, its additional effect cannot be produced. On the other hand, when the amount exceeds 30,000 ppm, the discharge capacity is rather lowered. The reason for this fact cannot yet be clarified, however, it is supposed as follows.
When a proper amount of the element of the above alkali metal or alkaline earth metal is adsorbed by or coated onto the surfaces of graphite powder, the metallic ions (cations) are electrically connected to negatively charged sites of the surface active effect material on the surfaces of graphite powder, so that the surfaces of graphite powder are electrically stabilized. Accordingly, the occlusion and releasing of lithium ions are facilitated, at the same time, the irreversible formation of lithium compound is suppressed.
As the method for adding further the above element to the graphite powder which adsorbs or coated with the surface active effect material, it is possible to add further the above-mentioned alkali metal or alkaline earth metal to the water to be used. More particularly, at least one member of the group consisting of inorganic acid salts such as hydroxides, chlorides, sulfides, bromides, oxides, iodides, sulfates, carbonates, nitrates, thiosulfates, acetates, perchlorides, citrates, tetraborates, oxalates, phosphates, lactates, sulfites, tartrates, nitrites, iodates of lithium, calcium, magnesium, sodium and potassium; or the salts of starch derivatives having a basic structure of C6H10O5, the salts of viscous polysaccharides having a basic structure of C6H10O5, the salts of water-soluble cellulose derivatives having a basic structure of C6H10O5 and the salt of water-soluble acrylic resin, the salt of water-soluble epoxy resin, the salt of water-soluble polyester resin and the salt of water-soluble polyamide resin. In addition, it is also possible to use ion-exchanged water, hot-spring water, under ground water, well water and city water which contain any one of lithium, calcium, magnesium, sodium and potassium.
By using one of these water, the treatment and operation like the above-described process of coating or adsorption with the surface active effect material to obtain the graphite powder containing the above element. In other words, because these elements exist in water in the form of ions, they are contained in the graphite material together with the surface active effect material to obtain the graphite powder of the present invention.
By the way, the drying after the filtration must be done at a temperature in which the material adsorbing or coated with the surface active effect material is not thermally decomposed. According to the use of the graphite powder, when aqueous paint or other additive is usable, such additive can be added to the treating solution.